Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the velocity of the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 4 as comprising a number of data tracks 6 defined by servo sectors 20-2N recorded around the circumference of each data track. Each servo sector 2i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 2, further comprises groups of servo bursts 14 (e.g., A, B, C and D bursts), which comprise a number of consecutive transitions recorded at precise intervals and offsets with respect to a data track centerline. The groups of servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
A conventional technique for writing the servo sectors 20-2N to the disk comprises writing a plurality of spiral servo tracks to the disk using an external spiral track servo writer, and then to servo off of the spiral servo tracks in order to write the servo sectors 20-2N. During the spiral servo track writing process, thermal expansion of disk drive components, such as the disk, head stack assembly, and spindle motor may cause unequal spacing of the spiral servo tracks resulting in radial position errors as well as phase errors when servo writing the product servo sectors 20-2N.